When a gaseous fluid (gas) is provided for mixing with another gas such as air, and then the mixture is to be used in a process of some kind, it is necessary to be able to control flow of the gas, and for this purpose valves are provided in the pipe or duct in which the gas flows. In these cases, it is usually necessary that the valve be capable of positively and reliably shutting off when gas flow is to end. These valves are mechanical devices and as such can malfunction for a number of reasons such as excess stress, vibration, particles in the gas stream, wear, or defects in manufacturing, any of which can result in leakage when the valve purportedly is shut. This is always undesirable, and may be a hazard if the gas involved has potential for doing harm if leakage occurs. Therefore it is important that leakage be reduced to an absolute minimum. However, it is not possible totally to prevent gas leakage in every valve of a large installed base because of the certainty of eventual deterioration and defects for a small fraction of valves, as well as the certain knowledge that the humans who have responsibility for proper operation of the gas control valves will not always perform their duties of inspection, use, and maintenance without error. For all these reasons, it is evident that means for sensing gas leakage in a valve provide an extra measure of safety and economy in the operation of such valves.
Perhaps the most common instance of gas flow control arises in the use and control of gaseous fuels (of which natural gas and propane are examples) used for heating and for industrial purposes. A very common situation is in gas fired burner systems where the burner is run for a period of time and then shut off when sufficient heat has bee provided. The gaseous fuel is mixed with air in a proportion and manner allowing for efficient combustion and heat generation. Such systems have frequent valve operation, and accordingly there is over a long period of operation, a non-zero probability of valve malfunction. While the remainder of this patent description will deal with sensing leakage of such gaseous fuels, the reader should understand that the description can apply to any situation where gas flow must be shut off on occasion and where the leakage of gas through the valve when shut is undesirable.
It is necessary for such systems using gaseous fuels regardless of their size to have their sequence of operation controlled, with each individual step necessary for safe and efficient operation of the heating unit occurring at the proper time. At the same time, there are a number of tests of operation and function which must be performed at preset times in the sequence to assure that previous steps have been performed properly. The newest versions of these systems use a microprocessor connected to control the elements of the heating unit. The microprocessor is programmed to command the sequence of the various functions which must be performed prior to, during and following an actual combustion operation.
These combustion systems typically include a combustion chamber, a source of the pressurized gaseous fuel, an air duct for carrying a flow of combustion air to the combustion chamber, a fuel injection nozzle within the air flow for supplying the gas to the combustion air so as to permit mixing of the fuel and air prior to entering the combustion chamber, and a valve for regulating flow of the fuel. Larger types of these systems have combustion air induced into the combustion chamber by use of a blower. There is a pipe which conducts the fuel from its source to the inlet port of the valve and another pipe which conducts the fuel from the outlet port of the valve to the nozzle.
It is customary in large systems to use a modulating fuel valve which can be opened to a number of different positions. In a system where there is a microprocessor which controls combustion system activity, such valves typically are electrically controlled by a solenoid receiving a valve control signal from the microprocessor. Modulating the valve between its closed and full open position controls the amount of fuel provided to the combustion chamber, and hence the rate of heat output. By adjusting speed of the blower or the position of dampers within the air duct the amount of air and fuel can be controlled so as to maintain almost precisely the stoichiometric fuel-air ratio. When there is no longer a demand for heat, the microprocessor sets the valve control signal to a closure value or signal which commands the valve to close completely, and the closure signal is maintained until another demand for heat occurs. For safety's sake, the valve is typically held closed by a spring against which the solenoid acts when opening the valve. Thus the closure signal state of the valve control signal may well be nothing more than the absence of electrical power to the solenoid.
As mentioned above, it is important that a valve responds to the closure signal by reliably, promptly and completely shutting so that unsafe amounts of gas cannot pass to the nozzle after the closure signal has been applied to the valve. In the past, there have been various design approaches to assure that such gas valves close reliably. For example, frequently two valves are used serially so that malfunction of both valves is necessary before fuel leakage occurs. It is common to specific periodic intervals for checking valve performance or to specify a service life after which the valve must be replaced. However, these approaches are relatively expensive and simply reduce the likelihood of valve leakage rather than allowing immediate correction of the leakage whenever it happens.
Another approach is to use devices which sense the presence of leaks and signal an operator when leakage is detected. There ar basically three different approaches which these type of devices use. A first senses flow within the fuel supply pipe when the valve is supposed to be closed, allowing the inference of a fuel leak. A second senses fuel pressure, inferring leaks from a change in pressure somewhere within the fuel delivery system. Neither of these techniques have the ability to sense potentially dangerous leaks in large systems where large amounts of fuel flow while the valve is open, because even a small amount of fuel flow on a percentage of maximum basis can be a relatively large amount of fuel in absolute terms.
A third type of leak detector relies on sensing presence of fuel within the system downstream of the fuel valve when the valve is receiving its closure signal. U.S. Pat. No. 3,999,932 describes a system using pressure buildup resulting from a gas leak in the control valve to close an auxiliary valve and shut down gas flow to the system. Of course, malfunction of the auxiliary valve or a failure of pressure buildup may allow leaks to occur. U.S. Pat. No. 4,375,353 discloses the use of a catalytic ga detector to detect presence of gas leaking into the combustion chamber of a furnace. The known characteristic of catalytic detectors to change their output with age or exposure to certain compounds may affect the reliability of their leak detection function.
Recently developed semiconductor devices called "microbridges" can accurately measure both the thermal conductivity and specific heat of gases, from which can be inferred the concentration of fuel in a fuel-air mixture. These sensors use highly stable noble metals and refractory materials as the elements in direct contact with the gas on which the measurements are performed. Such sensors typically include for example a pair of thin film temperature transducers adjacent a thin film heater, with the gas to be measured occupying a space between them. Semiconductor sensors of this type are discussed in more detail in one or more of U.S. Pat. Nos. 4,478,076; 4,478,077; 4,504,144; 4,651,564; and 4,683,159, all having an assignee common with the present application.
It known that the specific heat and thermal conductivity of gaseous fuels commonly used in burners today are substantially different from these properties for air. When the fuel is mixed with air, these properties of the resulting mixture differ from those of either pure fuel or pure air and are a function of the concentration of fuel in the air. Accordingly, it is known that by measuring either or both of these aforementioned properties one can determine the concentration of the gaseous fuel in air if the type of gaseous fuel involved is known.